1. Field of the Invention
The present invention relates to a transmission diversity technique for spread spectrum communication, and, more particularly, to an adaptive transmission diversity apparatus capable of determining a transmission radiation pattern in accordance with a reception radiation pattern.
2. Description of the Related Art
A transmission diversity technique for use in a conventional time division multiplex system and a frequency multiplex system will be described below with reference to FIGS. 1 and 2. FIG. 1 presents a block diagram showing the structure of a conventional adaptive transmission diversity apparatus, and FIG. 2 is a diagram illustrating the relationship between the incoming directions of radio waves and a reception radiation pattern.
Referring to FIG. 1, S.sub.1 (t), S.sub.2 (t), S.sub.3 (t) and S.sub.4 (t) denote complex signals, which have respectively been received at reception antennas 1, 2, 3 and 4 at time t and have then undergone A/D conversion and quasi-coherent detection. The outputs W.sub.1 (t), W.sub.2 (t), W.sub.3 (t) and W.sub.4 (t) of a reception radiation pattern controller 13 are respectively sent to multipliers 9, 10, 11 and 12 which in turn multiply the associated complex signals by the respective outputs. The multiplication outputs are then composed by an adder 14. The output, S(t), of the adder 14 then is given by the following formula (1). ##EQU1##
The above process of multiplying signals, received at a plurality of antennas, by the proper complex numbers and then adding the resultant values allows the antennas as a whole to acquire radiation pattern on a plane. When a desired signal is coming from the direction of an arrow 31 in FIG. 2 and an interference signal from the direction of an arrow 32, for example, the reception radiation pattern controller 13 in FIG. 1 controls the radiation pattern as indicated by reference numeral "33" in FIG. 2, so that the desired signal can be received at a strong level and the interference signal at a weak level. This control can enhance the reception performance.
A determination section 16 outputs a result D(t) of determining the composed signal S(t). An error detector 15 outputs a difference S(t)-D(t) between the composed signal S(t) and the determination result D(t). The reception radiation pattern controller 13 renew its output complex number weights W.sub.1 (t), W.sub.2 (t), W.sub.3 (t) and W.sub.4 (t) based on the output of the error detector 15 and the complex signals S.sub.1 (t), S.sub.2 (t), S.sub.3 (t) and S.sub.4 (t).
Given that the reception signal vector is given by Sig(t)=(S.sub.1 (t), S.sub.2 (t), S.sub.3 (t), S.sub.4 (t)).sup.T, the outputs of the reception radiation pattern controller, W(t)=(W.sub.1 (t), W.sub.2 (t), W.sub.3 (t), W.sub.4 (t)).sup.T, can be expressed by the following formula (2). And .mu. is coefficient of step. EQU W(t+1)=W(t)+.mu.(S(t)-D(t)).sup.T Sig(t) (2)
A transmission radiation pattern controller 17 computes weight outputs for transmission in consideration of a frequency difference between transmission and reception, etc. based on the outputs of the reception radiation pattern controller 13. Multipliers 22, 21, 20 and 19 multiply the outputs of the transmission radiation pattern controller 17 by a signal from a transmission signal generator 18. Antennas 23, 24, 25 and 26 convert the signals from those multipliers to RF (Radio Frequency) band signals, and transmit the resultant signals.
The above-described conventional time division and frequency division transmission diversity apparatuses have difficulty in detecting a directly arriving reception wave and an indirectly arriving reception wave reflected by buildings, mountains or the like and separating them from each other taking time shift into consideration. This makes it difficult to form a radiation pattern for each incoming wave, which results in a difficulty in controlling the transmission power with the radiation pattern that corresponds to the received wave.